1. Field of Invention
This invention relates generally to electric motor control circuits. More specifically, the invention relates to electric motor control circuits which set an upper limit on the rotation speed of electric motors and adjust the rotation speed within the set upper limit.
2. Description of Related Art
It is known to provide an electric motor control circuit which sets an upper limit on the rotation speed of an electric motor and adjusts the rotation speed within the upper limit. In this regard see U.S. Pat. No. 4,734,629, the teachings of which are incorporated herein by reference as if fully set forth, and the concept of which is explained in FIGS. 6 and 7.
FIG. 6 is a schematic diagram of the configuration of a known electric motor control circuit. FIG. 7 is a schematic diagram of a part of the electric motor control circuit shown in FIG. 6.
This known electric motor control circuit has a triac Q7 for controlling a voltage applied to motor M from an interchange power supply 50. A transistor Q6 provides gate pulse signals to triac Q7. A tachometer-generator TG detects the rotation speed of the motor M. An upper limit rotation speed setter S10 (see FIG. 7) sets the upper limit of rotation speed of the motor M. A trigger switch S9 adjusts the rotation speed of the motor M within a range having the upper limit. A phase control IC 60 controls a gate pulse signal outputting timing from transistor Q6.
The signal indicative of rotation speed of the motor M detected by tachometer-generator TG is input to a terminal P30, and converted into a voltage signal corresponding to the rotation speed of the motor M via a frequency/voltage converter 61 (refer to FIG. 7). A signal indicative of the upper limit of rotation speed output from setter S10 is input to a terminal P31. The voltage of both signals are compared by an operational amplifier 62 built in phase control IC 60, and a pulse signal indicative of this comparative result is input to a pulse timing setting circuit 63. A sawtooth signal which synchronizes with the interchange power supply 50 is input to the pulse timing setting circuit 63 via a terminal P41, and an agreement point of the voltage of this sawtooth signal and the voltage of the pulse signal is detected.
At the detected agreement point, an operation signal is output from the pulse timing setting circuit 63 to transistor Q6 via terminal P39. A gate pulse signal (trigger current) is output to triac Q7 from transistor Q6, and triac Q7 turns on, and the phase of the voltage applied to motor M is controlled.
Phase control IC 60 watches the rotation speed of motor M and controls the phase of the voltage applied to motor M to maintain the upper limit rotation speed set by setter S10.
The output of operational amplifier 62 is connected to trigger switch S9 via a terminal P34 and a transistor Q5. The voltage level of the output signal of operational amplifier 62 is changed by operating this trigger switch S9.
By operating trigger switch S9, the output timing of the pulse signal in the pulse timing setting circuit 63 is adjusted, and, the rotation speed of the motor M is adjusted within the range that the rotation speed doesn""t exceed the upper limit rotation number.
In this known electric motor control circuit, however, trigger switch S9 is connected to the output side of the operational amplifier 62 as shown in FIG. 7. The output level of the operational amplifier 62 is adjusted directly by the trigger switch S9. Accordingly, even if the rotation speed of the motor M decreases due to a load and the decreased number is detected by operational amplifier 62, the rotation speed of the motor M can""t be increased because the output of operational amplifier 62 is lowered by trigger switch S9.
That is, as shown in FIG. 8 showing a relationship between a load applied to motor M and the rotation speed of motor M controlled by the existing control circuit, if trigger switch S9 is set to the maximum stroke and the rotation speed reaches the upper limit rotation number, the rotation speed of motor M is not slowed by a certain load because the output level of operational amplifier 62 does not increase. Whereas the rotation speed is slowed by a load until the rotation speed reaches the upper limit rotation.
Specially, in an electromotive tool such as a sander and a polisher, because required rotation speeds of the motor are different due to finishing stages and polished parts, the rotation speed of the motor is changed frequently. In the above mentioned existing circuit, unless the stroke of the trigger is made longer, the rotation speed of the motor falls when a pad for grinding is put on a grinding side. When the rotation speed of the motor is changed, an established value of the upper limit rotation speed setter S10 must be adjusted, which is inconvenient. Moreover, it is desirable that the rotation speed of the motor can be changed by adjusting the stroke of the trigger, because delicate finishing is carried out with delicately changing the rotation speed of the motor in the grinding operation.
It is accordingly an object of the present invention to provide an electric motor control circuit which can prevent lowering of the rotation speed of a motor when a load is applied in the case where rotation speed is adjusted within a range that doesn""t exceed a predetermined upper limit.
This invention provides an electric motor control circuit meeting this objective. The electric motor control circuit has: an electric motor driven by an interchange power supply, a semiconductor control element having a gate that conducts when a pulse signal is applied to the gate and which causes, in turn, a voltage supplied from the interchange power supply to be applied to the electric motor. A pulse signal output means outputs the pulse signal to the semiconductor control element. A rotation speed detection means detects the rotation speed of the electric motor. An upper limit rotation speed setting means sets an upper limit for the rotation speed of the electric motor. A rotation speed adjustment means adjusts the rotation speed of the electric motor within a range that doesn""t exceed the upper limit rotation speed set by the upper limit rotation speed setting means. Comparative means compares a setting signal indicative of the upper limit rotation speed output from the upper limit rotation speed setting means with a detecting signal indicative of the rotation speed of the electric motor detected by the rotation speed detection means, and outputs a comparative signal which shows a comparative result of both signals. Output timing control means controls an output timing of the pulse signal output means based on the comparative signal output from the comparative means. The rotation speed setting means adjusts a signal level of the setting signal output from the upper limit rotation speed setting means.
The comparative means compares the setting signal which indicates the upper limit rotation speed output from the upper limit rotation speed setting means with the detecting signal which indicates the rotation speed of the electric motor detected by the rotation speed detection means, and outputs the comparative signal which shows the comparative result of both signals. The output timing control means controls the output timing of the pulse signal of the pulse signal output means based on the comparative signal output from the comparative means.
In other words, the comparative means detects the difference between a feed-back rotation speed of the motor and the upper limit rotation speed set up by the upper limit rotation speed setting means. And the output timing control means adjusts the rotation speed of the motor to maintain the upper limit rotation number, based on the detected difference.
Because the rotation speed setting means adjusts the signal level of the setting signal output from the upper limit rotation speed setting means, and the comparative means compares the setting signal of which the signal level is adjusted by the rotation speed setting means with the detecting signal showing the rotation speed of the motor. The rotation speed of the electric motor is maintained in the rotation speed shown by the setting signal of which the signal level is adjusted.
Because the level of the comparative signal output from the comparative means isn""t adjusted, the function of the comparative means and the output timing control means which maintains the rotation speed of the electric motor isn""t lost.
The setting signal output from the upper limit rotation speed setting means is adjusted, thus the rotation speed of the electric motor never exceeds the upper limit rotation speed by the adjusted level of the setting signal.
In one preferred embodiment, the rotation speed adjustment means has a variable resistor, and a transistor having a base connected to the variable resistor which controls the setting signal. The transistor can be used as a switch for the rotation speed adjustment. Since the transistor turns on, the signal level of the setting signal is adjusted so that the rotation speed of the electric motor can be adjusted within the range which doesn""t exceed the upper limit of rotation speed.
In another preferred embodiment, the electric motor control circuit has a delay circuit which gradually increases a base voltage applied to the transistor. Because of the delay circuit, rapid change in the base voltage is prevented when the adjustment of the rotation speed of the electric motor is started by the rotation speed adjustment means. Therefore, any sudden change of the signal level of the setting signal and any sudden increase of the rotation speed of the electric motor is prevented.
In another preferred embodiment, the electric motor control circuit has switching means for switching a voltage applied to the variable resistor between a voltage supplied to the upper limit rotation speed setting means and a voltage set up by the upper limit rotation speed setting means. In the case where the switch means switches the voltage applied to the variable resistor to the voltage supplied to the upper limit rotation speed setting means, the voltage applied to the variable resistor is changed within the voltage supplied to the upper limit rotation speed setting means. In the case where the switch means switches the voltage applied to the variable resistor to the voltage set up by the upper limit rotation speed setting means, the voltage applied to the variable resistor is changed within the voltage set up by the upper limit rotation speed setting means.
For example, it will be mentioned later, by connecting a switch S4 corresponding to the switching means to a terminal P20, the voltage applied to a trigger switch S3 corresponding to the variable resistor is switched to the voltage supplied to a dial switch S2 corresponding to the upper limit rotation speed setting means. By connecting the switch S4 corresponding to the switching means to a terminal P19, the voltage applied to a trigger switch S3 is switched to a voltage set up by dial switch S2.
In the case where the switch S4 is connected to the terminal P20, it will be understood with reference to FIG. 2(B) which shows relation between the rotation speed N of the motor M and a stroke L of the trigger switch S3, for example, when the upper limit rotation speed is set up in a level 4, until the stroke L of the trigger switch S3 becomes L4, the rotation speed N of the motor M linearly rises, and when the stroke L of the trigger switch S3 exceeds L4, the rotation speed becomes constant. In the case where the switch S4 is connected to the terminal P19, the rotation speed of the motor M linearly rises in proportion to the stroke L of the trigger switch S3 as shown in FIG. 5(B).
In other words, rising patterns of the rotation speed of the motor M can be changed by operating the switch S4.